1. Field of the Invention
This invention relates to a polarized light coherent combining laser apparatus. This invention particularly relates to a polarized light coherent combining laser apparatus, which is capable of combining a plurality of laser beams with one another and thereby obtaining a combined laser beam having a high energy.
2. Description of the Prior Art
As an apparatus for recording information, such as characters, on a recording material by utilizing a light beam, a laser computer output microfilmer (hereinafter referred to as a laser COM) has heretofore been proposed. With the laser COM, a laser beam is scanned in accordance with the information fed out of a computer, and the information, such as characters, is thereby directly recorded on a recording material, such as a microfilm. (The laser COM is described in, for example, U.S. Pat. No. 4,293,202.) The laser COM comprises an argon laser for producing a laser beam, an optical modulator for optically modulating the laser beam in accordance with the information, which represents characters, or the like, a rotating polygon mirror for deflecting the laser bream, which has been modulated by the optical modulator, in a main scanning direction, and a galvanometer mirror provided with a deflecting mirror for deflecting the laser beam, which has been reflected by the rotating polygon mirror, in a sub-scanning direction. With the combination of the rotating polygon mirror and the galvanometer mirror, the laser beam, which has been radiated out of the optical modulator, is two-dimensionally scanned on the recording material via a scanning lens. In this manner, the information, such as characters, is recorded on the recording material.
The laser COM described above utilizes the argon laser, which cannot be subjected to on/off control, and therefore it is necessary to provide the optical modulator. Accordingly, it has recently been proposed to utilize a semiconductor laser in lieu of the argon laser. However, in cases where semiconductor lasers are caused to oscillate continuously, the output power of the semiconductor lasers is as small as several milliwatts to several tens of milliwatts. Therefore, it is difficult for the semiconductor lasers to be applied to recording materials, which require a laser beam having a high energy, e.g., heat mode recording materials, such as laser direct recording films (LDFs).
Also, a technique for coherently combining the laser beams, which have been produced by a plurality of lasers, with one another by use of a diffraction grating and thereby producing a laser beam in a single polarized state has been disclosed in, for example, OPTICS LETTERS/Vol. 11, No. 5/May 1986.
However, the disclosed technique has the problems described below. Specifically, with the technique wherein the laser beams are combined with one another by using the diffraction grating, it is difficult to design the grooved surface configuration of the grating such that the direction of diffraction may coincide with a predetermined direction. Also, only the zero-order diffracted light component should be passed through an aperture, and the laser beams having been diffracted in nonessential directions should be blocked. Therefore, the efficiency of the optical system cannot be kept high.
Further, the laser beams, which have been produced by two semiconductor lasers, have heretofore been combined with each other by a polarizing beam splitter. However, the combined laser beam contains a P-polarized light component, which oscillates parallel to the plane of incidence upon the polarizing beam splitter, and an S-polarized light component, which oscillates in the direction normal to the plane of incidence upon the polarizing beam splitter. Therefore, if a polarizing element is located in the optical path of the combined laser beam, one half of the amount of light cannot pass through the polarizing element.
Accordingly, the applicant proposed a light amplifying device, which has a simple construction and can efficiently radiate out a laser beam having a high energy, in U.S. Pat. No. 5,048,030.
The proposed light amplifying device comprises:
i) an optical resonator constituted by first and second reflecting mirrors and a common reflecting mirror, which are located in optically opposite relation to each other, the common reflecting mirror having a reflectivity smaller than the reflectivities of the first and second reflecting mirrors,
ii) an optical path changing means, which is located between the first and second reflecting mirrors and the common reflecting mirror for making the laser beams incoming from a side of the common reflecting mirror emerge in a direction toward at least one of the first and second reflecting mirrors corresponding to either direction of an orthogonal plane of polarization, and for making laser beams incoming from the sides of the first and second reflecting mirrors incident upon the common reflecting mirror,
iii) first and second amplifying media, which are located in optical paths of the laser beams between the first and second reflecting mirrors and the optical path changing means, respectively, for amplifying the laser beams through the process of stimulated emission, and
iv) an optical element, which is located between the common reflecting mirror and the optical path changing means such that the optical element may be capable of transmitting the laser beams therethrough, and which rotates the plane of polarization of the laser beam having been reflected by the common reflecting mirror by a predetermined angle with respect to the plane of polarization of the laser beam coming from the optical path changing means.
With the proposed light amplifying device, the optical path changing means is located between the first and second reflecting mirrors and the common reflecting mirror, which are located in optically opposite relation to each other and which jointly constitute the optical resonator. The optical path changing means makes a laser beam incoming from the side of the common reflecting mirror emerge in a direction toward at least one of the first and second reflecting mirrors corresponding to either direction of the orthogonal planes of polarization. Also, the optical path changing means makes the laser beams incoming from the sides of the first and second reflecting mirrors incident upon the common reflecting mirror. The first and second amplifying media are located in optical paths of the laser beams between the optical path changing means and the first and second reflecting mirrors, respectively. Therefore, the laser beams coming from the optical path changing means are amplified through the respective amplifying media and then reflected by the first and second reflecting mirrors. Thereafter, the reflected laser beams are amplified again by the amplifying media and impinge upon the common reflecting mirror via the optical path changing means. The optical element is located between the common reflecting mirror and the optical path changing means. The optical element rotates the plane of polarization of the laser beam, which has been reflected by the common reflecting mirror, by a predetermined angle with respect to the plane of polarization of the laser beam coming from the optical path changing means. The direction of the optical path of the laser beam, which has passed through the optical element and again enters the optical path changing means, is changed to the direction toward at least one of the first and second reflecting mirrors in accordance with the direction of the plane of polarization of the laser beam having been rotated by the optical element. In this manner, the laser beam is caused to impinge upon the respective amplifying media.
In cases where the polarized light beams having the planes of polarization orthogonal to each other are P-polarized light and S-polarized light, the optical element serves to convert part or the whole of the P-polarized light into the S-polarized light and serves to convert part or the whole of the S-polarized light into the P-polarized light during the repeated reflection cycles of the polarized laser beams. Therefore, the optical resonator, which is constituted of the first reflecting mirror, the common reflecting mirror, and the amplifying medium located therebetween, and the optical resonator, which is constituted of the second reflecting mirror, the common reflecting mirror, and the amplifying medium located therebetween, jointly constitute a single united resonator, and the P-polarized light and the S-polarized light are thereby coherently combined with each other. Therefore, the P-polarized light and the S-polarized light are unified into a laser beam, which has a high energy and is in a single polarized state. In cases where the laser beams to be combined together have a phase difference therebetween, elliptically polarized light will ordinarily be obtained. In such cases, a combined laser beam in a single linearly polarized state can be obtained by compensating for the phase difference with a wavelength plate, or the like.
Recently, in the fields of optical fiber communication and optoelectronics, various attempts have been made to utilize a two-dimensional array of laser beam sources. The two-dimensional array comprises a plurality of laser beam sources, which are located in a two-dimensional pattern, such that a plurality of laser beams may be radiated out of the two-dimensional array to the direction normal to the two-dimensional array and may thereby be transmitted.
However, U.S. Pat. No. 5,048,030 does not indicate anything about a configuration for combining the laser beams radiated out of a two-dimensional array of laser beam sources. Thus with the light amplifying device disclosed in U.S. Pat. No. 5,048,030, a plurality of laser beams radiated out of a two-dimensional array of laser beam sources cannot be coherently combined with one another to yield a laser beam having a high energy.
In particular, if the laser beams produced by the respective laser beam sources constituting a two-dimensional array can be combined with one another, a laser beam having a high energy can be obtained with a compact configuration.